The air mass flow method estimates the heat transferred to an air stream from the temperature differential between inlet and outlet conditions. The air flow rate is taken either from equipment nameplate data or from direct duct measurements (area × velocity). The heat flow rate is then scaled to annual energy use using operating hours, the quantity multiplier, and system efficiency.

When using measured data:

\begin{equation}\label{eq:natural-gas-reduction-air-flow-measured} \dot{V}_{air} = A_{duct} \cdot v_{air}\end{equation}

Heat flow from the air stream:

\begin{equation}\label{eq:natural-gas-reduction-air-heat-flow} \dot{Q}_{air} = \frac{1.08 \cdot \dot{V}_{air} \cdot (T_{out} - T_{in})}{1{,}000{,}000}\end{equation}

Annual energy use:

\begin{equation}\label{eq:natural-gas-reduction-air-energy-use} Q_{total} = \frac{\dot{Q}_{air} \cdot t_{op} \cdot k_{units}}{\eta}\end{equation}

Symbols

\(\dot{V}_{air}\)	Air flow rate \([\unit{ \cubicFoot\per\minute}]\)
\(A_{duct}\)	Cross-sectional area of the duct \([\unit{ \squareFoot}]\)
\(v_{air}\)	Air velocity in the duct \([\unit{ \foot\per\minute}]\)
\(\dot{Q}_{air}\)	Heat flow rate from the air stream \([\unit{ \MMBtu\per\hour}]\)
\(1.08\)	Sensible heat factor for air (0.018 Btu/ft³·°F × 60 min/hr) \([\unit{ \unitless}]\)
\(T_{out}\)	Outlet air temperature \([\unit{ \degreeF}]\)
\(T_{in}\)	Inlet air temperature \([\unit{ \degreeF}]\)
\(Q_{total}\)	Annual natural gas energy use \([\unit{ \MMBtu\per\year}]\)
\(t_{op}\)	Annual operating hours \([\unit{ \hour\per\year}]\)
\(k_{units}\)	Quantity multiplier (number of identical units) \([\unit{ \unitless}]\)
\(\eta\)	System efficiency \([\unit{ \unitless}]\)